2008ApJ...680.1053C

We present a kinematic study of the outer regions (R₂₅<R<2R₂₅) of 17 edge-on disk galaxies. Using deep long-slit spectroscopy (flux sensitivity ∼few 10^–19 erg/s/cm2/arcsec2), we search for Hα emission, which must be emitted at these flux levels by any accumulation of hydrogen due to the presence of the extragalactic UV background and any other local source of UV flux. We present results from the individual galaxy spectra and a stacked composite. We detect Hα in many cases well beyond R₂₅ and sometimes as far as 2R₂₅. The combination of sensitivity, spatial resolution, and kinematic resolution of this technique thus provides a powerful complement to 21 cm observations. Kinematics in the outer disk are generally disklike (flat rotation curves, small velocity dispersions) at all radii, and there is no evidence for a change in the velocity dispersion with radius. We place strong limits, a few percent, on the existence of counterrotating gas out to 1.5R₂₅. These results suggest that thin disks extend well beyond R₂₅; however, we also find a few puzzling anomalies. In ESO 323-G033 we find two emission regions that have velocities close to the systemic velocity rather than the expected rotation velocity. These low relative velocities are unlikely to be simply due to projection effects and so suggest that these regions are not on disk-plane, circular orbits. In MCG -01-31-002 we find emission from gas with a large velocity dispersion that is corotating with the inner disk.